To assess the extent to which ACEA S.p.A. should utilize hybrid bonds relative to S&P’s 15% cap, we need to consider its financial position, capital structure, interest rate environment, and market conditions for hybrids.

**1. Capital Structure and Leverage**  
From the 2022 annual report data:  
- Total equity (2023-01-01): €2,755,243,000  
- Total non‑current financial liabilities (mostly debt): €4,722,263,000  
- Current financial liabilities: €619,418,000  
→ Total debt ≈ €5,341,681,000  
→ Total adjusted capital (debt + equity) ≈ €8,096,924,000  

15% of total adjusted capital = €1,214,538,600. ACEA’s existing equity is already well above that amount, so there is significant room under the cap to issue hybrids if needed.

**2. Profitability and Interest Coverage**  
- Operating profit (2022): €565.85m  
- Finance costs (2022): €111.67m  
- Profit before tax: €497.94m  
Interest cover (operating profit / finance costs) ≈ 5.07x, which is healthy.  
The company has solid cash flow from operations (€726.70m in 2022), with CapEx at high levels (€1.05bn for tangible + intangible). This indicates a need for funding, possibly partly through hybrids to preserve credit metrics.

**3. Interest Rate and Hybrid Market Conditions**  
The swap curves show a sharp rise in rates in 2022 (e.g., 5‑year from -0.264% in 2021 to 1.726% in 2022).  
The iBoxx EUR Non‑Financial IG Sub‑Sen delta 0.2 yield rose from 1.298% (2021) to 2.295% (2022).  
Higher rates make hybrid issuance more expensive. However, ACEA’s existing finance costs are still relatively low (€111m on ~€5.3bn debt, ~2.1% average cost). Hybrids would carry a higher coupon, but the equity credit benefit might offset the rating impact.

**4. Rating Considerations**  
S&P’s 15% cap is a maximum; utilising it fully (100%) is usually only for companies seeking aggressive equity credit to support leverage metrics while issuing debt. ACEA’s debt‑to‑equity is ~1.94x (debt/equity), and net debt/EBITDA would need to be calculated. Given the large CapEx programme and moderate leverage, some hybrid issuance could be useful, but fully maxing the 15% cap would likely be excessive.  
A partial utilisation (e.g., 50%) would give flexibility without over‑relying on expensive hybrid capital or risking future downgrades if cash flows weaken.

**Conclusion**  
A moderate use of hybrids (50% of the cap) would strike a balance—obtaining equity credit for rating support while not overleveraging with costlier subordinated debt in a rising rate environment.

<answer>50%</answer>